Distributed feedback semiconductor laser

ABSTRACT

This invention relates to a distributed feedback semiconductor laser which achieves distributed feedback by gain coupling by providing periodical changes in the thickness of the active layer (5) or the absorptive layer, and aims to achieve light distributed feedback mainly of periodical perturbation of the gain factor by diminishing the periodical perturbation of refractive index caused by the changes in the thickness of the active layer (5) or the absorptive layer. This invention is characterized by a refractive index canceling structure comprising a combination of layers (6) and (7) of different refractive indices to cancel periodical changes in refractive index caused by the periodical structure of the active layer (5) or the absorptive layer.

TECHNICAL FIELD

This invention relates to a distributed feedback semiconductor laserwhich is used as an electro-optic converter. This invention relates,more particularly, to a gain-coupled distributed feedback laser diode(GC-DFB-LD).

This invention is highly suitable as the light source for along-distance and large-capacity optical communication system, anoptical information processing system, an optical memory system, anoptical measuring instrument and other opto-electronic devices.

BACKGROUND TECHNOLOGY

The distributed feedback semiconductor laser generates stimulatedemission of light by distributed optical feedback to an active layerwith a diffraction grating provided near the active layer. The devicecan produce stimulated emission of excellent lasing spectralcharacteristics by a relatively simple construction, Therefore, it hasbeen the target of various R & D efforts and is expected to be used as alight source suitable for a long-distance and large-capacity opticalcommunication system, an optical information processing system, anoptical memory system and an optical measuring instrument.

Such distributed feedback semiconductor laser has an optical waveguidestructure, wherein an active layer is surrounded with transparenthetero-junction semiconductor layers, for efficient stimulated emission.R & D efforts are recently directed toward distributed feedback of lightby periodically changing the refractive index in a transparent opticalwaveguide layer which is placed very close to the active layer. In thatcase, a diffraction grating, having triangular cross section forinstance, is formed on the interface of the optical waveguide layer onthe side farther from the active layer.

In the light distributed feedback by such index coupling, however,feedback to phase cannot be matched for the light in Bragg wavelengthwhich is reflected correspondingly to the period of the thickness changein the optical waveguide layer. Because of this phase match condition,stable lasing cannot be obtained and two longitudinal lasing mode whosewavelengths are separated symmetrically in the vertical direction acrossthe Bragg wavelength may possibly be generated at once. Even if only onesuch longitudinal-mode-lasing takes place, it is difficult to selectpreviously which of the two wavelengths would be lasing. So, theprecision in setting lasing wavelength is seriously deteriorated.

In sum, the light distributed feedback using index coupling which isbased on the periodical perturbation of the refractive index in theoptical waveguide layer has an inherent problem of degeneracy bylongitudinal lasing mode of two wavelengths, which is difficult toavoid.

There have been proposed various solutions for the problem. One of themproposed a structure to shift the phase by 1/4 wavelength substantiallyat the center of the diffraction grating. Those proposals, however, arenot quite effective as they tend to make the construction of a lasermore complicated, require additional manufacturing steps only for thesolution of the degeneracy and need formation of anti-reflection coatingon the facets of the laser.

Kogelnik et al. proposed a theory in their paper titled "Coupled-WaveTheory of Distributed Feedback Lasers", Journal of Applied Physics,1972, Vol. 43, pp. 2327-2335, whereas a stop band is produced around theBragg frequency when distributed feedback of the light is conducted byindex coupling, if distributed feedback is conducted by gain couplingbased on the periodical perturbation of gain factors, such stop bandwould not be produced and longitudinal mode lasing of exclusively singlewavelength would be obtained. They did not mention in their paper thespecific construction to realize the theory, rather they merelydiscussed on the result of their theoretical studies.

Some of the present inventors have invented novel semiconductor lasersapplying the basic theory of Kogelnik et al., and filed patentapplications as follows:

Japanese Patent Application No. 63-189593, filed on Jul. 30, 1988

Japanese Patent Application No. 1-168729, filed on Jun. 30, 1989(Publication No. JP-A 3-34489)

Japanese Patent Application No. 1-185001 to 1-185005 filed on Jul. 18,1989) (Publication No. JP-A 3-49283 to 3-49287)

The inventors succeeded in realizing distributed feedback by gaincoupling by the invention constructions which were indicated in therespective specifications and drawings. Many of the constructions shownin these patent applications are provided with periodical corrugation onthe surface of an active layer to utilize the periodical perturbation ofgain factors caused by the changes in thickness.

The refractive index of the active layer is usually different from thatof the surrounding layers as it is necessary to confine the light. Ifthe active layer is corrugated, the refractive index inevitably changesperiodically. In other words, the construction having a corrugatedsurface on the active layer did not achieve the distributed feedback bygain coupling only, but was still subject to the effect of perturbationcaused by the index coupling.

Some of the present inventors therefore proposed a design that woulddiminish the perturbation caused by the index coupling so as to obtainthe perturbation caused by the gain coupling alone in the paper, "Purelygain-coupled distributed feedback semiconductor lasers", by Y. Luo, Y.Nakano, K. Tada, T. Inoue, H. Hosomatsu and H. Iwaoka, Appl. Phys. Lett.56 (17), Apr. 23, 1990, pp. 1620-1622. According to the proposedconstruction, the thickness of an active layer is periodically changedto provide gain coupling components, and the perturbation of therefractive index due to the corrugated surface of the active layer iscanceled by the refractive index perturbation of another corrugationprovided nearby with the opposite phase. The GC-DFB-LD which does notsubstantially contain the index coupling components is herein referredto as a "pure GC-DFB-LD".

Because the overflow of carriers from the active layer should beinhibited, materials of the layers on both sides of the active layershould have sufficiently wide band gap compared to that of the activelayer. Such materials, however, have low refractive index, and thereforetend to change the magnitude of the perturbation of refractive indexsensitivity to the forms of the two corrugations, for instance, thetooth height of the two corrugations. In order to effectively cancel theperturbation of refractive index, an extremely high manufacturingprecision is required.

In the GC-DFB-LD of GaAs based materials which has been realized so far,it was not considered to be a problem because high reproducibility bothin their diffraction gratings and the growth shape was relatively easilyrealized with AlGaAs. But when index coupling components are attemptedto be completely eliminated in a device of GaAs based materials, or whenthe material is inferior in manufacturing precision for canceling theperturbation of refractive index, the process should be controlledthoroughly and strictly.

It is an object of this invention to provide a distributed feedbacksemiconductor laser which can solve the above mentioned problems of theprior art, which can diminish distributed feedback caused by indexcoupling and which can obtain the distributed feedback caused mainly bygain coupling.

DISCLOSURE OF THE INVENTION

The first aspect of this invention provides a distributed feedbacksemiconductor laser comprising a layer of lower refractive index thanthat of the active layer provided at the peaks of the corrugation, alayer of an intermediate refractive index provided adjacent to thecorrugation, and a layer having a refractive index which is higher thanthat of the layer of lower refractive index and lower than that of theactive layer. The corrugation is provided on the surface of the activelayer as a diffraction grating.

The refractive indices of the active layer, theintermediate-refractive-index layer and the lower-refractive-indexlayer, the depth of the corrugation and the thickness of theintermediate-refractive-index layer are preferably as determined as tocancel the periodic changes of the refractive index caused by the activelayer and the intermediate-refractive-index layer by the periodicchanges of the refractive index caused by the lower-refractive-indexlayer and the intermediate-refractive-index layer.

In order to manufacture such a laser diode, an active layer is grown ona substrate, then a lower-refractive-index layer having a refractiveindex lower than that of the active layer is grown thereon. Thelower-refractive-index layer and the active layer are etched to have aperiodical corrugation as a diffraction grating. Then anintermediate-refractive-index layer having a refractive index higherthan that of the lower-refractive-index layer and lower than that of theactive layer is grown.

By adjusting the refractive index and the thickness of theintermediate-refractive-index layer to correspond to the forms of thediffraction grating etched on the lower-refractive-index layer and theactive layer, the components of index coupling can be controlled withprecision.

The distributed feedback semiconductor laser obtains the periodicperturbation of gain factors by periodically changing the thickness ofthe active layer.

By providing a low-refractive-index layer at each peak of thecorrugation on the surface of the active layer, and by further providingan intermediate-refractive-index layer adjacent to the corrugation, aperiodic structure is formed at the valleys of the corrugationcomprising the active layer, the intermediate-refractive-index layer,the active layer, the intermediate-refractive-index layer and itsrepetition. Another periodic structure is formed at the peaks of thecorrugation comprising the lower-refractive-index layer, theintermediate-refractive-index layer, the lower-refractive-index layer,intermediate-refractive-index layer and its repetition. The refractiveindices of those parts will become as below:

On the valley side:

High (active layer)-intermediate-high-intermediate . . .

On the peak side:

Low-intermediate-low-intermediate . . .

In other words, the periodic constructions on the valley side and on thepeak side become reverse in the phase to one another, whereby theperiodical changes of the refractive indices thereof can be canceled.

This restricts the periodic perturbation of index coupling, enablesdistributed feedback mainly of the periodic perturbation of gain factorswhich is caused by the periodic changes in the thickness of the activelayer and produces stable single mode lasing.

The second aspect of this invention aims to provide a distributedfeedback semiconductor laser comprising an active layer to generatestimulated emission, and a absorptive layer provided near the activelayer and made of a material which absorbs the stimulated emission oflight from the active layer. The absorptive layer has periodicallychanging thicknesses to give distributed feedback of the stimulatedemission from the active layer. It is characterized in that the periodicchanges in the refractive index, which is caused by the changes in thethickness of the absorptive layer, is canceled with a combination oflayers of different refractive indices.

According to the first aspect of the invention, the thickness of theactive layer is periodically changed and the perturbation of refractiveindex is canceled by the combination of different refractive indices.According to the second aspect of this invention, by changingperiodically the thickness of the absorptive layer instead of the activelayer, gains are changed effectively. Further, the perturbation ofrefractive index caused by the changes in thickness of the absorptivelayer is canceled by the combination of layers of different refractiveindices. This eliminates components of index coupling, and distributedfeedback can be achieved substantially by the changes in the opticalabsorption or the changes in the effective gain alone.

The construction for obtaining distributed feedback by gain coupling byproviding a periodical absorptive layer near the active layer isdiscussed in detail in the paper: "Fabrication and Characteristics of aGain-coupled Distributed-feedback Laser Diode," by Y. Luo, Y. Nakano andK. Tada, Extended Abstracts of the 20th (1988 International) Conferenceon the Solid State Devices and Materials, Tokyo, pp. 327-330.

The layer construction for canceling the periodic changes in therefractive index preferably includes a lower-refractive-index layerwherein its thickness changes in the same phase as the periodic changeof thickness of the absorptive layer but which has a refractive indexlower than the absorptive layer, and an intermediate-refractive-indexlayer wherein its thickness changes in the opposite phase to theperiodic change of the thickness of the absorptive layer and whoserefractive index is between that of the absorptive layer and that of thelower-refractive-index layer.

In order to cancel the index coupling component with due considerationto the weight of electric field intensity, the refractive indices,thicknesses and duty ratios of the absorptive layer, theintermediate-refractive-index layer and the lower-refractive-index layershould respectively be determined to satisfy either one of the followingconditions:

(1) The periodic changes in refractive index caused by the absorptivelayer and the intermediate refractive index layer are canceled by theperiodic changes in refractive index caused by thelower-refractive-index layer and the intermediate-refractive-indexlayer.

(2) The periodic changes in refractive index caused by the absorptivelayer and the lower-refractive-index layer are canceled by the periodicchanges in refractive index caused by the lower-refractive-index layerand the intermediate-refractive-index layer.

In addition to these conditions, in the case where a higher orderdiffraction grating is used, the refractive indices, thicknesses andduty ratios of the absorptive layer, the intermediate-refractive-indexlayer and the lower-refractive-index layer could respectively bedetermined to satisfy either one of the following conditions where thephases of periodic changes in refractive index are shifted from theconditions mentioned above:

(3) The periodic changes in refractive index caused by the absorptivelayer and the intermediate-refractive-index layer are canceled by theperiodic changes in refractive index caused by theintermediate-refractive-index layer and the lower-refractive-indexlayer.

(4) The periodic changes in refractive index caused by the absorptivelayer and the lower-refractive-index-layer are canceled by the periodicchanges in refractive index caused by the intermediate-refractive-indexlayer and the lower-refractive-index layer.

In these cases, the absorptive layer and the lower-refractive-indexlayer are preferably arranged close to or adjacent to each other. Theintermediate-refractive-index layer may be arranged on the opposite sideof the lower-refractive-index layer across the absorptive layer or onthe opposite side of the absorptive layer across thelower-refractive-index layer.

In the arrangement such as above, the imaginary part of the couplingcoefficient, or the gain coupling coefficient is generated by absorptionby the absorptive layer and will not be canceled. Therefore, distributedfeedback of the light is achieved substantially by gain coupling only.

It is preferable to fabricate the thinner portion of the absorptivelayer as thin as possible in order not to increase the loss caused bythe optical absorption unnecessarily. It is further preferable tosegment the absorptive layer so that the thicker portion, or the portionwhere the absorptive layer exists, concentrates corresponding to thenodes of the standing wave.

The referred combination of refractive indices which can cancel theperturbation of refractive index in respect of the first and secondaspects is selected in a manner to cancel the real part of the couplingcoefficient κ expressed by the equation below: The real part of thecoupling coefficient κ represents the component of index coupling. Moreparticularly, the coupling coefficient κ is represented by the followingequation given the direction vertical to the face of the diffractiongrating x, and the direction of optical propagation z,

    κ=(k.sub.0.sup.2 /2β.sub.0 P) ∫ A.sub.q (x) ε.sub.0 (x) ε.sub.0 * (x) dx

where:

k₀ : wave number in a free space

β₀ : propagation constant in z direction

A_(q) : component of the q-th order when the squares of the refractiveindices is Fourier expanded in the z direction

ε₀ : electric field intensity

P: constant obtained by integrating {ε₀ (x) ε₀ * (x)} in the x direction

In the case of the first order grating, the sign of the Fouriercoefficient A_(q) (x) is determined only by the sign of the differencein the squares of the refractive indices. Therefore the sign of thedifference in the squares of the refractive indices in the integrationis inverted, the coupling coefficient is canceled and becomesdiminished, and in an optimal case, the component of index coupling canbe designed as zero. In addition, in the case of higher orderdiffraction grating, the sign may also be inverted by the width per oneperiod, or the duty ratio. The design should take the duty ratio intoconsideration. The above equation is given to obtain the couplingcoefficient using Fourier expansion and is described in the paper by W.Striefer et al., IEEE J. Quantum Electronics QE-13, p. 134, 1977.

The word pertaining to "up" or "above" in this specification means thedirection identical to the direction of the growth of crystals at thetime of manufacture, or the direction away from the substrate, and theword pertaining to "below" means the direction opposite thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view to show the construction of the firstembodiment of this invention.

FIG. 2 is a cross section to show the layer structure in the vicinity ofthe an active layer.

FIG. 3 is a view to show the distribution of refractive indices alongthe line 3--3 in FIG. 2.

FIG. 4 is a view to show the distribution of refractive indices alongthe line 4--4 in FIG. 2.

FIG. 5 is a perspective view to show the structure of the secondembodiment of this invention.

FIG. 6 is a cross section along the stripe.

FIG. 7 is a cross section to show the structure of the third embodimentof this invention along the stripe.

FIG. 8 is a cross section to show the structure of the fourth embodimentof this invention along the stripe.

FIG. 9 is a cross section to show the structure of the fifth embodimentof this invention along the stripe.

OPTIMAL MODE TO REALIZE THE INVENTION

FIG. 1 shows the construction of an embodiment of this inventiondistributed feedback semiconductor laser.

The laser includes an active layer 5 for generating stimulated emissionof light, and a diffraction grating which is formed as corrugation onthe surface of the active layer 5 for achieving distributed feedback ofthe stimulated emission from the layer 5.

This invention is characterized in that a layer 6 having a refractiveindex lower than that of the active layer 5 is provided on each peak ofthe corrugation while an intermediate-refractive-index layer 7 having arefractive index higher than that of the layer 6 but lower than that ofthe active layer 5 is provided adjacent to the corrugation.

The construction as well as the manufacture method of the laser will bedescribed further in detail below. The InP based materials where thelayers are lattice-matched to InP will be described as an example.

Layers of double-hetero-junction structure are grown epitaxially in twostages on an n-type InP substrate of high carrier concentration. Thelayers are lattice-matched to the InP substrate 1.

In the first stage of epitaxial growth, on the substrate 1 aresequentially grown in crystals, for example, an n.type InP claddinglayer 3 of the thickness of 1 μm, an active layer 5 of In₀.53 Ga₀.47 Asof low impurity concentration of the thickness of 0.12 μm, and alower-refractive-index layer 6 of p-type InP of the thickness of 40 nm.Then, by the holographic exposure method and chemical etching, thelower-refractive-index layer 6 and the active layer 5 are etched to forma diffraction gratings (in corrugation) of the period of 256 nm and thedepth of 80 nm.

In the second stage of the epitaxial growth, anintermediate-refractive-index layer 7 of p-type InGaAsP of the averagethickness of 60 nm (the band gap λ_(g) =1.3 μm in terms of opticalwavelength) is grown further on the active layer 5 and thelower-refractive-index layer 6 which are etched with the diffractiongrating. The upper surface of the layer 7 is formed flat. Then, a p-typeInP cladding layer 8 of the thickness of 1 μm and a high carrierconcentration p-type In₀.53 Ga₀.47 As contact layer 9 of the thicknessof 0.5 μm are grown sequentially to complete the double-hetero-junctionstructure.

As the sides of the active layer 5 are exposed after the etching, it isnecessary to slightly etch them immediately before the crystal growth toavoid formation of defects. In the InP based materials there is nopossibility of defect if an appropriate processing is conducted, as isdiscussed in J. Cryst. Growth, 93 (1988) pp. 365-369.

After the second epitaxial growth, an SiO2 insulation layer 12 isdeposited on the upper surface of the contact layer 9 to form windows inthe form of stripes of the width of 10 μm, then electrode layers 10 and11 are evaporated. Then, the structure is cleaved into individualsemiconductor lasers.

The growth conditions in metal-organic vapor phase epitaxy (MOVPE) are,for examples, as below.

    ______________________________________                                        [materials]                                                                           phosphine      PH.sub.3                                                       arsine         AsH.sub.3                                                      triethylindium (C.sub.2 H.sub.5).sub.3 In                                     triethylgaliium                                                                              (C.sub.2 H.sub.5).sub.3 Ga                                     dimethylzinc   (CH.sub.3).sub.2 Zn                                            hydrogen sulfide                                                                             H.sub.2 S                                              [conditions]                                                                          pressure       76 Torr                                                        total flow rate                                                                              6 slm                                                          substrate temperature                                                                        700° C. (for the first stage of                                        growth)                                                                       650° C. (for the second stage                                          of the growth)                                         ______________________________________                                    

The conduction types and compositions of the above mentioned layers areshown below.

    ______________________________________                                        substrate 1        n.sup.+ -InP                                               cladding layer 3   n-InP                                                      active layer 5     i-In.sub.0.53 Ga.sub.0.47 As                               lower - refractive - index layer 6                                                               p-InP                                                      intermediate - refractive - index                                                                p-InGaAsP (λ.sub.g = 1.3 μm)                     layer 7                                                                       cladding layer 8   p-InP                                                      contact layer 9    p.sup.+ -In.sub.0.53 Ga.sub.0.47 As                        ______________________________________                                    

FIG. 2 shows the layer structure in the vicinity of the active layer ofthe aforementioned embodiment. FIGS. 3 and 4 show respectively thedistribution of refractive indices along the lines 3--3, 4--4 in FIG. 2.

The perturbation of the refractive index caused by the active layer 5with an etched diffraction grating and the intermediate-refractive-indexlayer 7 which fills in the etched dents is canceled by the perturbationof the refractive index caused in the reverse phase by the segmentedlower-refractive-index layer 6 and the intermediate-refractive-indexlayer 7 which fills in the cuts. The composition and thickness of thelayers mentioned above are one example of the design which could makethe perturbation of the refractive index substantially zero.

The upper surface of the intermediate-refractive-index layer 7 is madeflat to facilitate design calculations. If there are left someirregularities on the upper surface, the perturbation of the refractiveindex thereof should be taken into consideration in order to cancel theperturbation as a whole.

As is stated in the foregoing, a distributed feedback semiconductorlaser can be obtained which can diminish perturbation of the refractiveindex so as to realize distributed feedback mainly of the perturbationof the gain factor of the active layer 5 etched with a diffractiongrating to conduct single mode lasing in the Bragg wavelength whichcorresponds to the period of the gain factor.

FIG. 5 is a perspective view to show the construction of the secondembodiment of this invention distributed feedback semiconductor laser,and FIG. 6 is a cross section thereof along the stripe.

There is provided on a substrate 1 a cladding layer 3, an active layer 5which generates stimulated emission on the cladding layer 3. A layer forconfinement of carriers 14 is provided on the active layer 5, and alower-refractive-index layer 15 and an absorptive layer 16 aresequentially grown on the layer 14. An intermediate-refractive-indexlayer 17 is provided on the lower-refractive-index layer 16. On theintermediate-refractive-index layer 17, there are provided a claddinglayer 8 and a contact layer 9. An electrode 10 is connected to the lowersurface of the substrate 1 while an electrode 11 is connected to theupper surface of the contact layer 9 through a striped window providedon an insulation layer 12.

A periodic corrugation is formed on the upper surface of thelower-refractive-index layer 15 and the absorptive layer 16 is arrangedon the peaks thereof. This structure is obtained by epitaxially growingthe layers 15 and 16 flat and then etching them to segment the layer 16.It can also be obtained by growing the layers 15 and 16 whilecontrolling the crystal planes. The intermediate-refractive-index layer17 is formed in a manner to bury the segmented absorptive layer 16. Asthe layer 16 is periodically segmented, the absorption changes in theperiod and distributed feedback is realized.

The lower-refractive-index layer 15 and theintermediate-refractive-index layer 17 are designed to cancel theperiodical changes in refractive index caused by the changes in thethickness of the layer 16 by means of a combination of differentrefractive indices. In other words, the refractive indices and thethicknesses of the layers 15 and 17 respectively are determined asrelative to the refractive index and thickness of the absorptive layer16 so as to cancel the periodical changes in refractive index caused bythe layers 16 and 17 with the periodical changes in refractive indexcaused by the layers 15 and 17.

In the area where the layers 16 and 17 are alternately arranged on thesame plane or along the line A--A in FIG. 6 and where the layers 15 and17 are alternately arranged on the same plane of along the line B--B ofthe figure, the refractive index changes as below:

A--A: intermediate-high-intermediate-high-intermediate

B--B: intermediate-low-intermediate-low-intermediate

The refractive index is high in the absorptive layer 16. Therefore, thechanges in the refractive indices are canceled with one another. Theaccurate conditions for cancel are obtained according to the method ofcalculating the coupling coefficient by Streifer et al. The distributedfeedback can therefore be realized by changing the absorption or the netgain.

It is preferable to select the compositions for thelower-refractive-index layer 15 and the intermediate-refractive-indexlayer 17 to have sufficiently low optical absorption and yet to have therefractive indices close to that of the absorptive layer 16respectively. Taking an example of InP based materials, conduction typesand compositions are shown below:

    ______________________________________                                        substrate 1        n.sup.+ -InP                                               cladding layer 3   n-InP                                                      active layer 5     multi- quantum well (effec-                                                   tive λ.sub.g = 1.55 μm) layer of i-                                 InGaAsP (λ.sub.g = 1.3 μm) and                                      In.sub.0.53 Ga.sub.0.47 As                                 carrier confinement layer 14                                                                     p-InP                                                      lower - refractive - index layer 15                                                              p-InGaAsP (λ.sub.g = 1.3 μm)                     absorptive layer 16                                                                              p-InGaAsP (λ.sub.g =                                                   1.55 μm)                                                intermediate - refractive - index                                                                p-InGaAsP (λ.sub.g = 1.4 μm)                     layer 17                                                                      cladding layer 8   p-InP                                                      contact layer 9    p.sup.+ -In.sub.0.53 Ga.sub.0.47 As                        ______________________________________                                    

wherein: λ_(g) denotes the wavelength of the light corresponding to theband gap energy, and the quaternary alloy is lattice-matched to InP.

The active layer 5 and the absorptive layer 16 may be either the singlequantum well layers or the multi-quantum well layers. Alloys may beused. The positions of the absorptive layer 16 and the lower refractiveindex layer 15 are interchangeable.

FIG. 7 is a cross sectional view of the third embodiment of thisinvention shown along the stripe. The third embodiment differs from thesecond embodiment in the relative positions and the shapes of thelower-refractive-index layer 15, the absorptive layer 16 and theintermediate-refractive-index layer 17 respectively. More particularly,the intermediate-refractive-index layer 17 on which surface is formed aperiodic corrugation is provided on the carrier confinement layer 14. Alower-refractive-index layer 15 is formed on theintermediate-refractive-index layer 17 in a manner to reflect the effectof the corrugation on the surface. A absorptive layer 16 is furtherformed thereon in a manner to fill the dents in the corrugation and tosubstantially make the top surface flat. In other words, the thicknessof the absorptive layer 16 changes periodically.

In the area where the absorptive layer 16 and the lower-refractive-indexlayer 15 are alternately arranged within the same plane or along theline indicated at A--A and the area where the lower-refractive-indexlayer 15 and the intermediate-refractive-index layer 17 are arrangedalternately within the same plane or along the line indicated with B--Bin FIG. 7, the refractive index changes as follows:

A--A: high-low-high-low-high

B--B: low-intermediate-low-intermediate-low

Therefore, the changes in the refractive index are canceled.Alternatively, these two corrugated areas may be penetratingly combinedto make an area where the refractive index changes as follows:

-high-low-intermediate-low-high-low-intermediate-low-high

The coupling coefficient can be similarly calculated.

The conduction types and compositions of the layers in this embodimentmay be similar to those mentioned above.

FIG. 8 shows in section the fourth embodiment of this inventionstructure along the stripes. This embodiment of this invention structurealong the stripes. This embodiment differs from the third embodiment inthat the absorptive layer 16 is replaced with the lower-refractive-indexlayer 15. The operation thereof is similar to that of the thirdembodiment.

FIG. 9 shows in section the fifth embodiment of this invention structurealong the stripes. This embodiment differs from the second embodiment inthat the position of the portion comprising the lower-refractive-indexlayer 15, the absorptive layer 16 and the intermediate-refractive-index17 is replaced with the position of the active layer 5. In other words,on the cladding layer 3 are formed the lower-refractive-index layer 15,the absorptive layer 16 and the intermediate-refractive-layer 17.Further, the active layer 5 is formed thereon via a carrier confinementlayer 14. In this structure, the conduction types of some of the layersbecome opposite to those in the second embodiment. An example of theconduction types and compositions are shown below:

    ______________________________________                                        substrate 1        n.sub.+ -InP                                               cladding layer 3   n-InP                                                      lower - refractive - index layer 15                                                              n-InGaAsP (λ.sub.g = 1.3 μm)                     absorptive layer 16                                                                              n-InGaAsP (λ.sub.g =                                                   1.55 μm)                                                intermediate - refractive - index                                                                n-InGaAsP (λ = 1.4 μm)                           layer 17                                                                      carrier confinement layer 14                                                                     n-InP                                                      active layer 5     multi- quantum well (effec-                                                   tive λ.sub.g = 1.55 μm) layer of i-                                 InGaAsP (λ.sub.g = 1.3 μm) and                                      In.sub.0.53 Ga.sub.0.47 As                                 cladding layer 8   p-InP                                                      contact layer 9    p.sup.+ -In.sub.0.53 Ga.sub.0.47 As                        ______________________________________                                    

wherein λ_(g) denotes the light wavelength corresponding to the band gapenergy and the quaternary alloy is lattice-matched to InP.

In the third and fourth embodiments, the positions of the areacomprising the lower-refractive-index layer 15, the absorptive layer 16and the intermediate-refractive-index layer 17 and that of the activelayer 5 are interchangeable.

INDUSTRIAL APPLICABILITY

As described in the foregoing, the distributed feedback semiconductorlaser according to this invention has smaller changes of the refractiveindex as a whole to thereby diminish periodical perturbation of therefractive index. This enables distributed feedback mainly of theperiodical perturbation of gain factor which is caused by the periodicalchanges in the thickness of the active layer or the absorptive layer,which in turn enables lasing in single mode.

Unlike the prior art distributed feedback semiconductor laser of indexcoupling type, this invention distributed feedback semiconductor laserachieves distributed feedback by gain coupling, and is capable of lasingin completely single wavelength, and instability in lasing wavelengthwhich was often observed in the prior art is expected to be overcome.Completely single longitudinal mode lasing may be achieved by the priorart lasers, but it entails such inconveniences as that the structure ofthe semiconductor lasers becomes very complicated, and that the numberof manufacturing steps increases as it requires formation ofanti-reflection coating on the facet of a laser diode. This inventionlaser can realize single longitudinal mode lasing without the need tomodify the conventional manufacturing steps or to provideanti-reflection coating. Moreover, as this invention laser utilizesdistributed feedback by gain coupling, the noise caused by the externaloptical feedback from either a remote end or a adjacent end, if any, canbe remarkably reduced from those expected in the prior art lasers usingindex coupling.

As this invention laser can easily use quantum well layer as the activelayer, it can fully utilize their advantages. For example, a quantumwell active layer has the gain in TE mode larger than that in TM mode tothereby enable TE mode lasing selectively. The distributed feedbacksemiconductor laser according to this invention can generate very shortpulses in high speed current modulation and is expected to have lesschirping in the lasing wavelength.

The distributed feedback semiconductor laser according to this inventionis therefore not only very promising as the high performance lightsource necessary for long-distance and large-capacity opticalcommunication systems, but also is expected to be used as a small sizedlight source of high performance which can replace conventional gaslasers or solid lasers used in the fields of optical informationprocessing and recording systems, optical measuring instruments, and asthe light source for high speed optical phenomena.

When distributed feedback based on the perturbation of the absorption orperiodic changes of the net gain is used, components of index couplingcan be reduced as compared with the conventional structure which issimply added with a absorptive layer, and in the threshold gain.differences can be increased to thereby improve characteristics.

When compared with the structure wherein the components of indexcoupling are canceled by devising the forms in the vicinity of theactive layer having periodically changing thickness, manufacture of thepresent invention laser is facilitated in the following aspects.

Firstly, canceling of the components of index coupling becomes easier.This is because unlike the case where as active layer thickness ischanged, use of the absorptive layer lifts the restrictions on thecompositions of the lower-refractive-index layer and of theintermediate-refractive-index layer imposed by the problem of overflowof carriers, and the difference in refractive indices of the absorptivelayer and of the lower-refractive-index layer can be made smaller. Evenif there are minor variations in the forms, the magnitude of residualindex coupling due to the variations can be reduced by the decrement inrefractive index difference @ore precisely, difference in the squares ofrefractive indices), giving an allowance in the manufacturing precision.

Secondary, as the absorptive layer can be reduced in the thickness byselecting an the adequate composition to effectively absorb the light,the necessary depth in the corrugation can be reduced. This contributesto alleviating the conditions for etching the corrugation or forregrowth thereon.

Thirdly, as compared to the case where the active layer is corrugated,the active layer is less prone to damages. The absorptive layer is lessinfluenced by the problem of re-growth than the active layer.

This invention can be realized without being extremely limited inmaterials, and realize a pure GC-DFB-LD by using various materials suchas InP based materials in addition to GaAs based materials.

What is claimed is:
 1. In a distributed feedback semiconductor lasercomprising:an active layer to generate stimulated emission, a periodicalstructure to give light distributed feedback of stimulated emission fromthe active layer, and a refractive index canceling structure to cancelthe periodical changes in the refractive index caused by the periodicalstructure, wherein: said periodical structure is formed as corrugationon the surface of said active layer, and said refractive index cancelingstructure includes a lower-refractive-index layer having a refractiveindex which is lower than that of said active layer and provided on eachof a plurality of peaks of said corrugation, and anintermediate-refractive-index layer provided in contact with saidcorrugation and having a refractive index higher than that of saidlower-refractive-index layer and lower than that of said active layer.2. The distributed feedback semiconductor laser as claimed in claim 1wherein respective indices of the active layer, theintermediate-refractive-index layer and the lower-refractive-indexlayer, the depth of the corrugation and the thickness of theintermediate-refractive-index layer are determined in a manner to cancelthe periodical changes of refractive index caused by said active layerand said intermediate-refractive-index layer with the periodical changesof refractive index caused by said lower-refractive-index layer and saidintermediate-refractive-index
 3. In a distributed feedback semiconductorlaser comprising:an active layer to generate stimulated emission, aperiodical structure to give light distributed feedback of thestimulated emission from said active layer, a refractive index cancelingstructure to cancel the periodical changes in refractive index caused bythe periodical structure, and an absorptive layer formed near saidactive layer with a material which absorbs the stimulated emissiongenerated by the active layer, and wherein: said periodical structure isformed as changes in the thickness of the absorptive layer, and saidrefractive index canceling structure includes a layer in a locationwhere it can cancel the periodical changes in refractive index caused bythe changes in the thickness of said absorptive layer by an adequatecombination of layers of different refractive indices.
 4. Thedistributed feedback semiconductor laser as claimed in claim 3 whereinsaid layer structure for canceling the periodical changes in refractiveindex includesa lower-refractive-index layer whose thickness changes inthe same phase as the period of the thickness of the absorptive layerand which has a refractive index lower than that of the absorptivelayer, and an intermediate-refractive-index layer whose thicknesschanges in a phase opposite to the period of the thickness of saidabsorptive layer and which has a refractive index intermediate of saidabsorptive layer and said lower-refractive-index layer.
 5. Thedistributed feedback semiconductor laser as claimed in claim 4 whereinthe absorptive layer is arranged between the lower-refractive-indexlayer and the intermediate-refractive-index layer.
 6. The distributedfeedback semiconductor laser as claimed in claim 4 wherein thelower-refractive-index layer is arranged between the absorptive layerand the intermediate-refractive-index layer.
 7. The distributed feedbacksemiconductor laser combination as claimed in claim 3, wherein saidabsorptive layer is sufficiently near said active layer to absorb thestimulated emission generated by said active layer; and wherein saidperiodical structure is in the area of said emission.